I have a 12 ft jon boat I fish out of. I need to purchase a battery. Found a Marine Dual Purpose with a reserve capacity of 210... it was the biggest battery available. Is this battery adequate for a 50# thrust TM. What does the dual purpose mean?

The dual purpose just means that it can be used as a deep cycle or cranking battery.  A cycle for a battery is a discharge and then a recharge to full capacity.  As I'm sure you know, cranking batteries aren't meant to be fully discharged and this is why deep cycles, as their name implies, can withstand being almost fully depleted and can be recharged back to almost full capacity. 

 

You can read a lot of posts on batteries on BassResource and this has been a hot topic lately. You'll see that there are a lot of variables with battery life and electronics in general and there are no absoutes. Some variables include the weight of your boat and tackle, wind, current, battery age and conditions, etc. However, there are some very generic formulas you can use to get a 'ballpark' figure.  The reserve capacity is how long the battery will last at a 25 amp rate. A 50# trolling uses an approximate amp rate of 50 amps at full speed.  At 50 percent thrust you will be using a 25 amp rate.  This means at 50 percent thrust, you can get approximately 3.5 hours out of your battery (210 mins/60 minutes).  At full and contstant thrust you'll get half that. 

Keep in mind also, thats standing on the trolling motor non stop, Not adjusting position and fishing, full out running it.